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Thread: Resolution limited by diffraction?

  1. #1

    Resolution limited by diffraction?

    I'm throwing this up as a new thread, even though it arises out of a concurrent thread on Quickloads. (I don't want to disrupt the flow of deep philosophy dominating that series of exchanges...)

    On a side discussion of edge diffraction, Jorge noted, in part, "But.....when I was working with 4x5 I had some instances where difraction ruined my shots. For you and your printing method it might not be a consideration, but for others it might be. IMO it is good to be aware of the possibilities."

    I'm familiar with edge diffraction in theory, and am trying to make it have a theoretical impact in my head. Mind you, I'm just doing the math off the top of my head, and I'm an ignorant fool besides... But, assuming an extremely small f/stop of 1mm (that's f/64 on a 65mm lens, or f/90 on a 90mm lens), and a wavelenght of light around 500 nanometers, (visible light is 400-700 nm), the area along the circumference that would affect light (pi x .oo05, where pi is the circumference in mm and 0.0005 is the mm value of 500 nm) is 0.oo157 square mm out of a total aperture area of 0.785 square mm. That comes out to 1/500 of the passing light possibly being affected by edge diffraction of the aperture.

    I don't think 1/500 of the light being affected by diffraction would ruin an exposure. Anyone a bit more familiar with optics/physics/math care to clue me in where my logic is failing?

    BTW,I think my high school students changed my posting name to "William Mortensen", and I've been trying to change it back. We'll see how it comes out this time...

  2. #2

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    Resolution limited by diffraction?

    It takes some study of physical optics to derive diffraction equations. The results are:
    the resolution of a diffraction limited lens is about 1600/f in lines per mm for visible light. That is an f/16 lens can resolve about 100 lines/mm, at f/64 you are down to about 25 lines/mm. If you want 10 lines/mm in the print, then 10x12 is the largest print you can make from 4x5 at f/64. I've used f/64 for a 12x16 print, its just fine at "normal" viewing distance, but a little softer than my usual f/22.5.

  3. #3

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    Resolution limited by diffraction?

    diffraction limited is one of those terms which seems to be variable in that it depends on what you consider to be the acceptable size for the circle of confusion.

    download the free rodenstock software here and play with it to see where diffraction kicks in depending on what you set CoC to.

  4. #4

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    Resolution limited by diffraction?

    forgot to say that defraction limit is the point at which diffraction becomes more significant than other lens aberations in blurring the image. It doesn't mean image is ruined but rather that the required coc cannot be attained.

  5. #5

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    Resolution limited by diffraction?

    The "bottom line", if you will, is that with LF diffraction won't make a damned bit of difference in your outcomes compared to the myriad of inevitable, real world human errors that screw up images. Put your attention to those rather than diffraction.

    Now someone will jump in and say "Yeah, BUT if you enlarge to (some incredibly stupid size) AND (sniff grain) (and imposes stupidly irrelevant opitcal-bench-racing metrics) you will see the difference!

    But YOU ain't THEM. Pitty the grain sniffers.

    Are we okay now?

  6. #6

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    Resolution limited by diffraction?

    You seem to know a bit about the subject, but you also seem a bit off base. Read Jacobson's Lens Tutorial at www.photo.net. (You will find it under "Learn".) Diffraction theory says that the image of a point light source passing through a circular aperture is a central bright disc surrounded by concentric rings, with decreasing intensity from the center out. The rings are assumed to be too dark to affect everything, but the central Airy disc, not being a point, results in a blurry image of that point. The effect on the total image is to make it slightly blurry, uniformly over the whole image, indepdendent of other considerations. The formula for the radius of the Airy disc shows that its radius is dependent on the relative aperture (f-number) and the wavelength of light. It is usually calculated for a middle wavelength in the range where the eye is most sensitive.

    The combined effect of diffraction and defocus is pretty complicated, and Jacobson has some discussion of that fact.

  7. #7
    Abuser of God's Sunlight
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    Resolution limited by diffraction?

    "with LF diffraction won't make a damned bit of difference in your outcomes compared to the myriad of inevitable, real world human errors that screw up images"

    well, if you stop down real far, then it will make a difference everyone can see. it's hard to make a sharp looking enlargement from a neg made at f64 (all due respect to the f64 group). unless you consider stopping down too far to be a human error

  8. #8

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    Resolution limited by diffraction?

    well, if you stop down real far, then it will make a difference everyone can see. it's hard to make a sharp looking enlargement from a neg made at f64

    On what format, what degree of enlargement? I call bullshit. Of course, this raises the opportunity for all the sofa-racers to claim their eyes are better than the norm without having to show evidence. Let the bullshit flow.

    Spit the hook out of your mouth, paulr.

  9. #9

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    Resolution limited by diffraction?

    I used to have a very clear explanation from a post on diffraction limits from here by one of our acknowledged maths wizzes that set it all out very clearly and basically said you don't need to worry your pretty little heads about it until you get beyond at least f45 in 4x5 and f64 in 8x10.

    Always seemed pretty true in practice

  10. #10

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    Resolution limited by diffraction?

    Despite what I've heard and studied about the effects of aperture related diffraction also being inversely proportional to focal length, as the actual, physical opening of a given f-stop increases as focal length increases, I've not really noticed this in practice. What I do notice is that yes - f/64 and smaller openings can be a little soft - but often advantageously so. To me, every aperture has its "ideal" use. If depth isn't an issue, I'll usually go to f/22 with F.L.'s longer than 120, or to f/16 with shorter lenses. One might argue for an even wider aperture, but I've never quite trusted the degree of flatness of L.F. film - in whatever holder. One to two stops down (f/22-32 to f/32-45, depending on F.L.) offer balances of compromise between sharpness and depth, while f/64 and smaller openings are used either when I need as much depth as possible and there's no other way to achieve this, or because there is some other compelling reason, like working with movement over time or wanting a different optical signature - which might feel right with a given subject.

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